Fluorine and its compounds have widely been used for atomic power industry, medical products, household articles, and so forth due to its unique characteristics. Since a fluorine gas (F2 gas) is chemically stable and cannot be isolated by methods other than electrolysis, the fluorine gas is produced by electrolysis using an electrolytic bath containing a fluoride ion. Also, a several useful fluorine compounds are produced by electrolytic synthesis using the electrolytic bath containing fluoride ion. Among others, a nitrogen trifluoride gas (NF3 gas) has recently been increased in production amount like the fluorine gas.
Industrial-scale mass production of the F2 gas has been conducted so as to use the F2 gas as a raw material for synthesis of uranium hexafluoride (UF6) for uranium concentration and sulfur hexafluoride (SF6) for a high dielectric gas. In semiconductor industries, since the F2 gas reacts with a silicon oxide film or selectively reacts with an impure metal, the F2 gas is used for dry cleaning of silicon wafer surfaces. Also, as general industrial usages, the F2 gas is being used on industrial-scale as a fluorine processing raw material for suppressing a gas permeability of a high density polyethylene used for a gasoline tank or a fluorine processing raw material for improving wettability of an olefin-based polymer. When the olefin-based polymer is processed with a mixture gas of fluorine and oxygen, a carbonyl fluoride group (—COF) is introduced into a surface of the olefin-based polymer. The carbonyl fluoride group easily changes to a carboxyl group (—COOH) by hydrolysis such as a reaction with humidity in the air to improve the wettability.
The F2 gas was isolated by Moissan in 1886 for the first time, and then Argo et al. succeeded in synthesizing the F2 gas by electrolyzing a mixed molten salt of potassium fluoride and hydrogen fluoride in 1919, whereby the F2 gas synthesis industry was established. In the initial stage, a carbonaceous material such as graphite or nickel was used for an anode. Though nickel is usable also in an electrolytic bath containing water, it is rapid in corrosion and dissolution, has a current efficiency of about 70%, and is subject to a large amount of fluoride sludge. Therefore, a method that is employed most frequently at present is a method established in 1940s, wherein a carbon electrode is used as an anode and KF-2HF molten salt having a KF:HF molar ratio of 1:2 is used as an electrolytic bath. However, this method still has many problems in operation.
In the case of using the carbon electrode in the electrolytic bath containing a fluoride ion, such as the KF-2HF molten salt, a fluorine generation reaction represented by Formula (1) due to discharge of an fluoride ion occurs on a surface of the electrode, and, at the same time, graphite fluoride (CF)n having a C—F bonding of a covalent bonding property represented by Formula (2) is generated to cover the electrode surface. Since (CF)n is considerably low in surface energy, wettability thereof with the electrolytic bath is poor. Though (CF)n is thermally decomposed into carbon tetrafluoride (CF4) or ethane hexafluoride (C2F6) as represented by Formula (3) due to Joule heat, the carbon electrode surface is covered with (CF)n when a speed of Formula (2) exceeds that of Formula (3) to reduce an area for the electrode to contact an electrolytic solution, thereby ultimately stops a flow of a current. That is, a so-called anode effect is exhibited ultimately. When a current density is high, the speed of Formula (2) is increased to easily cause the anode effect.HF2−→½F2+HF+e−  (1)nC+nHF2−→(CF)n+nHF+e−  (2)(CF)n→xC+yCF4,zC2F6, etc.  (3)
The anode effect tends to occur when a water content in the electrolytic bath is high. As shown in Formula (4), carbon on the electrode surface reacts with water in the electrolytic bath to generate graphite oxide [CxO(OH)y]. Since CxO(OH)y is unstable, it reacts with atomic fluorine generated due to the discharge of fluoride ion as represented by Formula (5) to change into (CF)n. Further, due to the generation of CxO(OH)y, an interlayer gap of graphite is widened to facilitate diffusion of fluorine, thereby increasing the generation speed of (CF)n represented by Formula (2). Thus, it is apparent that the anode effect occurs easily in the case where a water content in a mixed molten salt bath containing the fluoride ion is high.xC+(y+1)H2O→CxO(OH)y+(y+2)H++(y+2)e−  (4)CxO(OH)y+(x+3y+2)F−→x/n(CF)n+(y+1)OF2+yHF+(x +3y+2)e−  (5)
The anode effect is a big problem in using the carbon electrode since the occurrence of the anode effect remarkably reduces a production efficiency, and an explosion can be caused in some cases if a power supply was not stopped immediately after the occurrence of the anode effect. Therefore, operation is complicated by the anode effect since it is necessary to perform water content control in the electrolytic bath employing dehydration electrolysis, and it is necessary to maintain a current density lower than a critical current density with which the anode effect occurs. The critical current density of generally used carbon electrodes is less than 10 A/dm2. Though it is possible to raise the critical current density by adding 1 to 5 wt % of a fluoride such as lithium fluoride and aluminum fluoride to the electrolytic bath, the critical current density can only be raised to about 20 A/dm2.
An NF3 gas was synthesized for the first time in 1928 by Ruff et al. by using a molten salt electrolysis and consumed by a large scale as a fuel oxidizing agent for a planetary exploration rocket planed and produced by NASA of U.S.A. to draw much attention. At present, the NF3 gas is used on a large scale as a dry etching gas in a semiconductor manufacturing process and a cleaning gas for a CVD chamber in a semiconductor or liquid crystal display manufacturing process. In recent years, since it has been clarified that a PFC (Perfluorinated Compound) such as carbon tetrafluoride (CF4) and ethane hexafluoride (C2F6) used for a cleaning gas for CVD chamber influences greatly on the global warming, the use of PFC is being restricted or prohibited internationally by the Kyoto Protocol, and the NF3 gas is used on a larger scale as a substitute for the PFC.
At present, NF3 is manufactured by two types of methods, i.e. by a chemical method and molten salt electrolysis. In the chemical method, F2 is obtained by electrolyzing the KF-2HF mixed molten salt, and then NF3 is obtained by reacting F2 with a metallic fluoride ammonium complex or the like. In the molten salt electrolysis, a molten salt of ammonium fluoride (NH4F) and HF or a mixed molten salt of NH4F, KF, and HF is electrolyzed to directly obtain NF3. In the case of using the mixed molten salt of NH4F, KF and HF, the NH4F—KF—HF molten salt of a molar ratio of 1:1:(2 to 5), respectively, is ordinary electrolyzed by using a carbon electrode as an anode. In this method, in the same manner as in the case of obtaining F2 by electrolyzing the KF-2HF molten salt, it is necessary to perform the complicated water content control in the electrolytic bath for the purpose of preventing the occurrence of the anode effect, and it is necessary to operate under the critical current density. Further, there has been a problem that CF4 and C2F6 generated by Formula (3) reduce a purity of the NF3 gas. Since properties of CF4 and properties of C2F6 or NF3 are remarkably close to each other, it is difficult to separate them by distillation, Therefore, there is another problem that, for the purpose of obtaining high purity NF3, it is inevitable to employ a purification method which is a cause of an increase in cost.
In the case of obtaining NF3 by using the NH4F—HF mixed molten salt, the NH4F—HF mixed molten salt having a molar ratio of 1:(1 to 3) is ordinarily electrolyzed by using nickel as an anode. In this method, it is possible to perform electrolysis using the electrolytic bath containing moisture as in the same manner as in obtaining the F2 gas by using the KF—HF mixed molten salt, and the method has an advantage of synthesizing NF3 which is not contaminated by CF4 and C2F6. However, since nickel is dissolved into an electrolytic solution to accumulate at the bottom of the electrolytic cell as a nickel fluoride sludge, it is necessary to change the electrolytic bath and the electrode at a constant interval, and it is difficult to produce NF3 continuously. An amount of dissolution of nickel reaches to 3 to 5% of a power supply. Since the nickel dissolution amount is remarkably increased when the current density is increased, it is difficult to perform electrolysis at a high current density.
As described in foregoing, there has been a strong demand for an anode material having properties of reduced in anode effect, sludge, and generation of CF4 in the electrolysis using an electrolytic bath containing a fluoride ion in order to continuously conduct a stable production.
Fluoride metallic gases are necessary for formation of a thin film, a dopant for ion implantation, and lithography in the semiconductor and liquid crystal display manufacturing processes, and many of the fluoride metallic gases are synthesized by using the F2 gas as a starting material. Therefore, the anode material having the above-described properties is in demand also for producing the fluoride metallic gases.
[Reference 1] JP-A-7-299467
[Reference 2] JP-A-2000-226682
[Reference 3] JP-A-11-269685
[Reference 4] JP-A-2001-192874
[Reference 5] JP-B-2004-195346
[Reference 6] JP-A-2000-204492
[Reference 7] Carbon; vol, 38, page 241 (2000)
[Reference 8] Journal of Fluorine Chemistry, vol. 97, page 253 (1999)
Among the above described carbon electrodes, the so-called electroconductive diamond electrode using electroconductive diamond as an electrode catalysis has been adapted to various electrolysis processes. Reference 1 proposes a processing method wherein an organic substance in a waste liquid is decomposed by oxidization using the electroconductive diamond electrode. Reference 2 proposes a method of chemically processing an organic substance by using the electroconductive diamond electrode as an anode and a cathode. Reference 3 proposes an ozone synthesis method using the electroconductive diamond electrode as an anode. Reference 4 proposes peroxosulfuric acid synthesis using the electroconductive diamond electrode as an anode. Reference 5 proposes a method of disinfecting microbes using the electroconductive diamond electrode as an anode.
In all of the above literatures, the electroconductive diamond electrode is applied to solution electrolysis containing no fluoride ion, and these inventions do not consider the electrolytic bath containing a fluoride ion.
Though Reference 6 discloses a method of using a semiconductor diamond in a bath containing fluoride ion, the invention relates to an organic electrolytic fluorination reaction by way of a fluorine substitution reaction caused after the dehydration reaction in a potential region lower than a potential at which the discharge reaction of fluoride ion represented by Formulas (1) and (2) occurs, i.e. in a region free tom a fluorine generation reaction, and it is impossible to apply the method to the productions of the fluorine gas and NF3. Therefore, when the electrode according to Reference 6 is used in the region of occurrence of the discharge reaction of fluoride ion, which inhibits stability of existent carbon electrodes and nickel electrodes and is represented by Formula (1), problems such as discontinuation of the electrolysis due to decay of the electrodes are caused.